1. Field of the Invention
The present invention relates to a reflective liquid crystal display device having a bright light display and high contrast among the characteristics of the bright light display, and, a wide angle of view on the display plane in vertical and horizontal directions, and superior visual characteristics.
2. Description of the Related Art
In general, liquid crystal display devices are either a semi-transmissive or transmissive type having a backlight, or a reflective type. Reflective liquid crystal display devices produce a display without a backlight by using only ambient light, such as sunlight or artificial light, and are widely used in, for example, portable information terminals or the like that need a low profile, light weight, and low power consumption.
FIG. 10 is a schematic cross-sectional view showing the configuration of a well-known type of reflective liquid crystal display device. In particular, the device in this example is a passive-matrix STN (Super-Twisted Nematic) type.
In this reflective liquid crystal display device, a liquid crystal cell 72 for a reflective STN mode and a retardation film 73 are placed in that order on the surface of a lower polarizing plate 70 of a reflection plate 71, and an upper polarizing plate 74 is further placed on the retardation film 73.
In the liquid crystal cell 72, a lower glass substrate 75, a color filter 76, a lower transparent electrode layer 78, a lower alignment layer 79, an upper alignment layer 80 placed opposing the lower alignment layer 79 with a space therebetween, an upper transparent electrode layer 81, and an upper glass substrate 82 are laid in that order on the surface of the lower polarizing plate 70. An STN liquid crystal layer 83 is interposed between the lower and upper alignment layers 79 and 80. An overcoat layer (not shown) made of silica or acrylic resin is provided between the color filter 76 and the lower transparent electrode layer 78.
The retardation film 73 compensates for the phase difference of light passing through the STN liquid crystal, thereby preventing the display from taking on a blue or yellow tint.
The liquid crystal display device is generally required to have high display performance, such as having high resolution, high contrast, a bright screen, vivid colors, high visibility, and a wide angle of view.
When the conventional reflective liquid crystal display device is applied to a display section of a portable information terminal or the like, however, a high-contrast region of the display screen in the horizontal direction is large, whereas a high-contrast region in the vertical direction is small. Therefore, the angle of view on the display screen in the vertical direction is narrow, the dependence on the angle of view is great, and visual characteristics are unsatisfactory. Moreover, light display (white display) is darker than that of a transmissive liquid crystal display device equipped with a high-intensity backlight.
Accordingly, it has been suggested that the white display during the application of a selection voltage be brightened by removing the lower polarizing plate 70 placed between the liquid crystal cell 72 and the reflection plate 71, and by using only the upper polarizing plate 74 on the retardation film 73. In such a reflective liquid crystal display device, however, removing one of the polarizing plates not only brightens the light display, but also brightens the dark display (black display), which results in a loss of contrast.